It is known that cements or silicates or aluminous hydraulic binders which are to be hydrated in compact forms are utilized because of their binding properties and because of their high strength. These properties are used by manufacturers of concrete and form products which possess properties of compactness, hardness, resistance to compression, and good binding qualities.
Studies have been carried out to determine how the cement becomes hydrated. During use, the grains of the cement become hydrated rapidly after a latent period of induction, the so-called setting period and, thereafter, the hydration proceeds progressively slower. The hydration obtained in this way proceeds with the formation of gels or finely intergrown crystals which solidify the medium by reducing the possible ionic movements. This restricts the chemical reactions so that the hydration of aluminous binders primarily includes hemi-and mono-calcium aluminates after six months or even one year. In the case of Portland cements, the hydration reactions can last tens of years.
It is known that hydrates of finely divided hydraulic binders are prepared in the laboratory in the presence of an excess of water with or without agitation and grinding. Typical examples of laboratory preparation can be found at pages 3682 to 3690 of "Journal of the Chemical Society", 1950 (H.S.W. Taylor) and at page 180 of "The Chemistry of Cement and Concrete" 3rd. Edition, F.M. Lea.
According to these processes, grains of anhydrous material are dispersed in water and may or may not be agitated. This material dissolves slowly and, through the chemical reaction, precipitates hydrates slowly. Thus, in the cited work of F.M. Lea at pages 180 and 181, it is reported that a complete hydration of tricalcium silicate can be obtained in a ball-mill, in the presence of water, within one or two days. Dicalcium silicate, an important constituent of Portland cement, is reported to take about 46 days to hydrate under those conditions.
Known ways of accelerating the process are generally of the type useful only in a laboratory and include physical means such as the use of elevated temperatures or known chemical means such as the use of accelerators, but these approaches are discontinuous and the hydrates obtained usually contain anhydrous material.
The applicants have carried out numerous tests and research projects and have realized that the hydrates are a combination of several ions arising from the anhydrides and water drawn from the reactive medium. The hydrates formed in the course of the hydration of the hydraulic binders are preferably formed at the surface of the least soluble grains of the anhydrides, that is, in the vicinity of which the solubility-product of the compound to be formed is attained rapidly. This indicates that the surfaces of the least reactive anhydrous particles are less and less in contact with the water and that, due to this fact, the kinetics of the reaction of hydration is self-diminishing and ultimately terminates our local reaction. In this connection, the applicants have observed that the particles become surrounded by a hydrated layer and the further reaction is governed by the ionic diffusion across this hydrate layer. This dissolution is governed by the well-known laws of Fick and, in the course of time, becomes negligible.
Known material which are used as mineral charges include kaolins, pyrogenic kaolins, natural and synthetic calcium silicates such as wollastonite, natural and synthetic calcium carbonates, talc, silica, dolomite, barium sulphate. Some of these materials are obtained in micronic form through washing and granulometric selection, such/as kaolin. Other materials are obtained through precipitation in solution, starting from ions obtained by dissolving highly soluble salts such as sodium silicates and sodium aluminates, lime, and aluminum sulphates. In addition, other materials are obtained through the grinding of natural rock by the wet or dry method to obtain the desired degree of fineness. This is the most common case and can be used to obtain micronic powders of carbonates, talcs, dolomites, and silicas to be used as charges or in charges. A fineness in the order of a micron is very expensive and very difficult to carry out through grinding operations.
In the co-pending patent application Ser. No. 520,101, there is disclosed a process for the manufacture of mineral charges of hydrates of calcium aluminate, aluminous cements, and Portland cements. The process starts with synthetic calcium aluminates as used in the manufacture of hydraulic binders or starting with anhydrous calcium silicates. The process of this co-pending patent application includes grinding up the material to an average degree of fineness in the order of one or several tens of microns and hydrating with a sufficient quantity of water to form a paste including hydrated simple and complex calcium aluminates alone or in mixture thereof with hydrated calcium silicate and subjecting at least the large particles to vigorous agitation during the hydration of the paste.
Hydrated particles in the order of a micron are obtained because the vigorous agitation breaks free the hydrated shell which forms around particles and therefore exposes surfaces for the reaction to form the hydration.
Although this process shows a considerable savings in the consumption of energy and reduces the time needed for hydration, there are many cases in which the requirement for vigorous agitation can create problems.
The instant invention makes it possible to obtain hydrated products of at least equal quality while avoiding some of the problems pointed out above.